The invention relates to air-conditioning circuits for motor vehicles.
In conventional motor vehicles, the compressor in the air conditioning circuit is driven by the engine and thus consumes some of the power from the engine. Although the power drawn by the compressor, when it is running, is not great, it does have an effect on the efficiency of the engine. The fact is that the power actually drawn by the compressor reduces the efficiency of the engine, thus increasing fuel consumption and the pollution generated by the vehicle""s exhaust gases.
To optimise the efficiency of the engine, one solution is to estimate the instantaneous power actually drawn by the compressor. With a knowledge of this information, it is in fact possible for the injection parameters of the engine to be adjusted to the actual requirements that exist. Without a knowledge of this information, the injection computer selects, by default, the injection parameters which correspond to the maximum figure for power drawn, a level which is rarely reached in practice.
This drawback may relate to internally controlled mechanical compressors which operate via a clutch inserted between the engine and the compressor. In the regulated mode, internally controlled compressors adjust their volumetric displacement by following a linear law which relates the level of the pressure at the input to the compressor, termed the low pressure, to the level at the delivery of the compressor, termed the high pressure. However, it does happen that the power actually drawn by the compressor is less than its nominal power.
Compressors of this kind draw a power which depends on the operating conditions and which therefore cannot be reduced even if the power actually drawn by the compressor is known. Conversely, it is possible for the operation of the air conditioning to be regulated by disconnecting the compressor when the power level is unacceptable.
This drawback is even more of a nuisance with externally controlled compressors, the use of which is becoming more widespread.
The fact is that in externally controlled mechanical compressors the power actually drawn by the compressor is often less than its nominal power. Consequently, the injection to the engine has to compensate for the difference between the nominal mechanical power and the mechanical power actually drawn, which reduces the efficiency of the engine.
In known embodiments, the instantaneous power drawn by the compressor is estimated from a map of the most frequently used states of operation. This map includes reference states, each reference state being associated with a value for the power drawn by the compressor which was provided by prior tests. The power drawn by the compressor is estimated by comparing the operating state of the air conditioning circuit with a reference state which forms part of the map. Methods based on mapping of this kind require long development times and are based on empirical data. They have the drawback of not taking into account all the possible circumstances of operation and hence of giving results which are only approximate.
In other embodiments, the instantaneous power drawn by the compressor is calculated from an estimate of the flow rate of refrigerant fluid. French patent application No. 01 16568 proposes estimating the flow rate of refrigerant fluid in the air conditioning circuit from an item of information relating to the speed of the vehicle and an item of information relating to the voltage from the motorised fan unit. However, these two items of information are not available on all vehicles.
It is an object of the invention to propose an air conditioning system which enables use to be made of an equation which relates the flow rate of refrigerant fluid to parameters relating to the refrigerant fluid which are available in any vehicle, to give an estimate of the flow rate of refrigerant fluid and in particular to enable the power drawn by the compressor to be calculated.
In more general terms, it is an object of the invention to propose an air conditioning system which enables use to be made of such an equation to give an estimate of a magnitude relating to the refrigerant fluid.
To this end, the invention proposes an air conditioning system for motor vehicles which is fitted with an injection computer and with a refrigerant fluid circuit comprising a compressor, a cooling member, an expansion member and an evaporator. The system also comprises an electronic control device intended to interact with the refrigerant fluid circuit and the injection computer. Advantageously, the system comprises:
a first measuring member suitable for supplying a value relating to the pressure of the fluid at a first point in the air conditioning circuit, termed the first pressure, said first point being situated between the outlet of the expansion member and the outlet of the evaporator,
a second measuring member suitable for supplying a value relating to the pressure of the fluid at a second point in the air conditioning circuit, termed the second pressure, the second point being situated at the inlet to the compressor, while the electronic control device is able to make use of the solution of an equation which relates the mass flow rate of the refrigerant fluid to the difference in pressure between the first point and the second point in order to calculate an estimate of a magnitude relating to the refrigerant fluid.
In a first embodiment of the invention, the magnitude relating to the refrigerant fluid is the mass flow rate of refrigerant fluid, while the electronic control device is able to solve the said equation from the value of the first pressure and the value of the second pressure.
In a second embodiment of the invention, the magnitude relating to the refrigerant fluid is the second pressure and the electronic control device is able to solve the said equation from the value of the mass flow rate of the fluid and the value of the first pressure.
In this second embodiment of the invention, the cooling member is a condenser and the system comprises measuring members suitable for supplying a value relating to the temperature of the flow of outside air at the inlet to the condenser and a value relating to the pressure of the fluid at the delivery of the compressor, termed the high pressure, while the electronic control device is able to use the values supplied by said measuring members in order to make use of the solution of an equation which relates the mass flow rate of the refrigerant fluid to the temperature of the flow of outside air at the inlet to the condenser and to the high pressure to calculate an estimate of the instantaneous value of the mass flow rate of the refrigerant fluid.
In a third embodiment of the invention, the compressor is of variable displacement and the magnitude relating to the refrigerant fluid is the minimum value of the pressure of the fluid corresponding to the maximum displacement of the compressor, the electronic control device being able to solve the said equation from the value of the first pressure and the value of the speed of rotation of the compressor.
The value of the speed of rotation of the compressor may be supplied to the electronic control device by the injection computer.
In a particular embodiment, the system comprises a compressor which is fitted with a control valve and the second measuring member is a sensor suitable for supplying the instantaneous value of current at the control valve, the electronic control device being able to calculate an initial estimate of the second pressure from the value of the current to the control valve of the compressor which is supplied by the second measuring member.
In this embodiment, the electronic control device is able to compare the initial estimate of the second pressure with the minimum value of the second pressure.
The electronic control device is then able to react to the fact that the initial estimate of the second pressure is equal to or less than the minimum value of the second pressure by supplying a final estimate of the second pressure which is substantially equal to the minimum value of the second pressure.
The electronic control device is likewise able to react to the fact that the second pressure is higher than the minimum value of the second pressure by supplying a final estimate of the second pressure which is substantially equal to the initial estimate of the second pressure.
In another embodiment, the second member is a sensor which is positioned at the second point and which is suitable for supplying the instantaneous value of the second pressure directly.
According to another feature of the invention, the first measuring member is a sensor which is positioned at the first point and which is suitable for supplying the instantaneous value of the first pressure.
In a first variant, the first measuring member may be a temperature probe which is positioned in the fins of the evaporator and which is suitable for supplying a measurement of the instantaneous value of the temperature of the air in the evaporator.
In a second variant, the first measuring member may be a temperature probe which is positioned downstream of the evaporator and which is suitable for supplying a measurement of the instantaneous value of the temperature of the air entering the evaporator.
In a third variant, the first measuring member may be a temperature probe which is suitable for supplying the instantaneous value of the temperature of the fluid, the probe being positioned at the first point, in contact with the liquid part of the fluid.
In this third variant, the first point is situated at a location in the air conditioning circuit at which the refrigerant fluid is in a diphase state.
In particular, in the first variant and the third variant, the expansion member may be a thermostatic expander, while the temperature probe is positioned in the zone where fluid is injected into the evaporator.
In these three variants, the electronic control device is able to estimate the value of the first pressure from the value supplied by the temperature probe.
As a supplementary provision, the temperature probe has a time constant equal to or less than 5 seconds.
Other features and advantages of the invention will become apparent from perusal of the detailed description below and from the accompanying drawings, in which: